An epoxy resin consists of polyether polymers, which depending on the reaction route with the addition of suitable hardeners produce a thermosetting plastic of high strength and chemical resistance. If epoxy resin and hardener are combined, the curing of the originally viscous mixture occurs typically within a few minutes to a few hours depending on the composition and temperature. Different systems are commercially available as hardeners for epoxy resins such as, for example, hardeners on an amine, amide, and anhydride base (for example, under the name Epikure® from Hexion Specialty Chemicals) or hardeners based on polyether amines (for example, under the name Jeffamine® from Huntsman).
Mannich bases of primary amines, formaldehyde, and phenols can also be used as hardeners or accelerators (cf., for example, EP 0 253 339 A). Further, WO 00/15687 also describes a Mannich base accelerator, which is prepared by transamination of a Mannich base with an amine.
A disadvantage of relevant customary Mannich bases is the use of phenol as a starting material, because the obtained Mannich bases often comprise not yet reacted phenol. Because of the toxicity of free phenol, phenol-based Mannich bases therefore cannot be used for many fields of application. For this reason, a great effort has been made to produce phenol-free Mannich bases. Thus, for example, Mannich bases based on nonylphenol or p-tert-butylphenol or cardanol were developed (cf. U.S. Pat. No. 6,262,148). Appropriate commercial products, such as Aradur® 3442 from Huntsman Advanced Materials Inc., are also known.
A further disadvantage with the use of customary Mannich bases is that these are prepared with use of formaldehyde. Formaldehyde is implicated, inter alia, in the development of allergies and irritation of the skin, respiratory tract, and eyes. There is a need accordingly to provide Mannich-like condensation products that can be prepared without use of formaldehyde.
Another major disadvantage of known customary Mannich hardeners is the high viscosity, arising during their preparation because of the formation of oligomers and secondary products. Thus, high-molecular-weight and relatively high-viscosity resoles form by the reaction of phenols with formaldehyde in a customary Mannich base synthesis under basic conditions. If these high-molecular-weight and relatively high-viscosity resoles are to be reacted, for example, with polymeric polyamines, which per se also have a higher viscosity, to form customary Mannich bases, this generally does not succeed because of the high viscosity of the resulting reaction mixture. Therefore, customary Mannich bases of phenols, formaldehyde, and polymeric polyamines are generally not obtainable.
During the preparation of customary Mannich base hardeners, a large excess of amine is typically used to keep the molecular weight and thus the viscosity low. Therefore, customary Mannich base hardeners are generally used in a mixture with typical polyamines. The admixing of additional polyamines, however, mostly has a negative effect on the properties of the cured epoxy resin composition.
Moreover, the processes for preparing known Mannich bases are very costly and difficult to run, particularly when the formation of high-molecular-weight condensation products is to be prevented if possible. Thus, for example, EP 1 475 411 A discloses a two-stage process for preparing Mannich bases based on m-cresol or 3,5-xylenol and polyamines, in which preferably a tertiary amine is used. Likewise, a two-stage process for preparing Mannich bases is disclosed in EP 1 475 412 A, whereby these are obtained from phenols such as m-cresol, 3,5-xylenol, or resorcinol with polyamines, preferably with the use of tertiary amines.
Such two-stage processes are associated with additional expenditures and increase the cost of preparing Mannich bases.
Proceeding from this state of the art, the object of the present invention is to provide alternative accelerators for epoxy resins or hardeners for epoxy resins and polyurethanes, which preferably do not possess the present disadvantages.
The object of the present invention in particular is to provide accelerators for epoxy resins or hardeners for epoxy resins and polyurethanes, which contain no free phenol.
The object of the present invention further is to provide accelerators for epoxy resins or hardeners for epoxy resins and polyurethanes, which contain no free formaldehyde.
Preferably, the use of phenols and formaldehyde should therefore be dispensed with in the preparation of the pertinent accelerators.
Furthermore, the object of the present invention in particular is to provide accelerators for epoxy resins or hardeners for epoxy resins and polyurethanes, which have no oligomeric byproducts.
Therefore, the accelerators should be obtainable in a simple manner without the formation of unwanted high-molecular-weight condensation products.